Conventionally, there has been publicly known a turbocharger bearing housing in which a cooling passage for circulating cooling liquid is formed. Such a turbocharger bearing housing is disclosed, for example, in Japanese Patent Application Laid-Open Publication No. Hei. 9-310620.
The turbocharger bearing housing disclosed in Japanese Patent Application Laid-Open Publication No. Hei. 9-310620 is manufactured by casing. Further, the turbocharger bearing housing disclosed in Japanese Patent Application Laid-Open Publication No. Hei. 9-310620 includes the cooling passage for circulating cooling liquid which is formed so as to surround the periphery of a bearing portion turnably supporting a shaft.
In such a conventional turbocharger bearing housing, since the shape of the cooling passage is complicated, normally when the bearing housing is formed by casting, the cooling passage is simultaneously formed by using a collapsible core formed from molding sand and a resin binder.
With reference to FIG. 9 and FIG. 10, description will be given of the disadvantageous points and the solution of the method for manufacturing such a conventional turbocharger bearing housing.
The conventional turbocharger bearing housing is manufactured by casting. As shown in FIG. 9A, a mold 160 used in the casting includes a casting main body portion 162 that is a cavity portion having substantially the same shape as a desired casting (specifically, the turbocharger bearing housing).
A collapsible core 150 for forming the cooling passage inside the bearing housing is disposed inside the casting main body portion 162. The collapsible core 150 includes a circulation forming portion 151, an end part forming portion 152, and a fixing portion 154. The circulation forming portion 151 is formed so as to surround the periphery of the bearing portion turnably supporting the shaft of the turbocharger. The end part forming portion 152 is connected to the vicinity of the upper end portion of the circulation forming portion 151, and is extended upward from the circulation forming portion 151 (to an upper portion of the casting main body portion 162). The fixing portion 154 is connected to the upper end of the end part forming portion 152, and is embedded in and fixed to the mold 160 (the upper portion of the casting main body portion 162) to thereby hold the end part forming portion 152 and the circulation forming portion 151 in a prescribed position. The circulation forming portion 151 and the end part forming portion 152 of the collapsible core 150 form the portions corresponding to the cooling passage formed inside the bearing housing.
In the case where a molten metal 70 is supplied (cast into a mold) via a weir 164 to the inside of the casting main body portion 162, in which the collapsible core 150 configured as above is disposed, of the mold 160, a moment of force is applied to the end part forming portion 152 of the collapsible core 150 by buoyancy applied to the circulation forming portion 151 when the circulation forming portion 151 of the collapsible core 150 is soaked in the molten metal 70 to a certain degree as shown in FIG. 9B. Thereby, the end part forming portion 152 is damaged (broken), which is a disadvantageous point that the bearing housing may not be manufactured.
To solve the above-described problem, as shown in FIG. 10, the method in which the collapsible core 150 is formed with an auxiliary fixing portion 155 is available. The auxiliary fixing portion 155 is connected to the vicinity of the lower end portion of the circulation forming portion 151, and is extended downward from the circulation forming portion 151 (to a bottom portion of the casting main body portion 162). The lower end portion of the auxiliary fixing portion 155 is embedded in and fixed to the mold 160 (the bottom portion of the casting main body portion 162).
The collapsible core 150 is thus formed with the auxiliary fixing portion 155 so that the circulation forming portion 151 is supported in the two directions, namely, the support manner of the collapsible core 150 is a both-end support manner (specifically, the circulation forming portion 151 is supported by the end part forming portion 152 from above, and is supported by the auxiliary fixing portion 155 from below). With this configuration, even if buoyancy is applied to the circulation forming portion 151, the circulation forming portion 151 is supported not only by the end part forming portion 152 but also by the auxiliary fixing portion 155, the collapsible core 150 can be prevented from being damaged.
However, the bearing housing obtained by the manufacturing method (casting method) using the mold 160 and the collapsible core 150 as shown in FIG. 10 is formed with not only holes of the end portion of the cooling passage (specifically, holes formed by the end part forming portion 152) but also unnecessary holes (specifically, holes formed by the auxiliary fixing portion 155). Accordingly, there is a disadvantageous point because of a necessity for inserting a plug so as to block the unnecessary holes.